1. Field of the invention
The present invention relates, in general, to refrigerating apparatus. More specifically, the invention relates to a refrigerating apparatus in which a microcomputer controls the capacity of a compressor in response to a refrigerating load to maintain the suction pressure of the compressor at a predetermined value.
2. Description of the prior art
Refrigerating apparatus including a microcomputer and an inverter circuit are well known. In this conventional refrigerating apparatus, the output of a compressor is controlled by the microcomputer through the inverter circuit in response to the changes of a refrigerating load.
An example of the above-described refrigerating apparatus is shown in FIG. 1. A refrigerating cycle 11 includes a compressor 13, a condenser 15, a liquid tank 17, an expansion valve 19, an evaporator 21, and an accumulator 23, all connected in series. Expansion valve 19 and evaporator 21 are arranged in a display case 25 to refrigerate food stored in the case. A suction pressure sensor 27 is provided at the suction side 13a of compressor 13. A detection signal from suction pressure sensor 27 is fed to a control circuit 29, such as a microcomputer. The microcomputer controls a driving frequency for compressor 13 through an inverter circuit 31 in response to the detection signal to maintain the suction pressure of compressor 13 at a prescribed value.
Generally, high efficiency of a refrigerating cycle is achieved by maintaining the temperature of an evaporator at a specific value calculated from the design specification of the refrigerating cycle. There is a specific relationship between the condensation temperature of refrigerant and the suction pressure of a compressor. If the suction pressure of the compressor goes down, the condensation temperature of the refrigerant falls.
In FIG. 1, if the suction pressure of compressor 13 is controlled to a prescribed pressure value, a high efficiency of refrigerating cycle 11 can be achieved. In this refrigerating cycle, the driving frequency for compressor 13 is varied in response to the difference between the prescribed pressure value of the suction pressure and the actual suction pressure value of compressor 13 detected by sensor 27. Thus, the actual suction pressure of compressor 13 is controlled to the prescribed pressure value, as shown in FIG. 2.
In FIG. 2, a plurality of frequency variation zones are determined in accordance with the deviation of the suction pressure of the compressor. A specific frequency value is set in each frequency variation zone. The driving frequency for compressor 13 is changed on the basis of the frequency value in the frequency variation zone correcting the pressure deviation of compressor 13. Thus, the actual suction pressure of compressor 13 is changed to the prescribed pressure value. During the refrigerating operation, if the suction pressure of compressor 13 is changed due to the change of a refrigerating load, the driving frequency for compressor 13 is changed on the basis of the frequency value in the corresponding frequency variation zone until the suction pressure of compressor 13 reaches the prescribed pressure value. If the suction pressure of compressor 13 falls below a suction pressure lower limit value Lv notwithstanding the control of the driving frequency for compressor 13, compressor 13 is stopped after the execution of the refrigerant pumping down operation.
During a refrigerating operation, a fluid mixture of refrigerant and lubricating oil is circulated through a refrigerating cycle by the compressor. When the compressor stops, the fluid mixture remains at its position in the refrigerating cycle. In particular, a large amount of refrigerant stays in the evaporator. This is because the dissolution of refrigerant into lubricating oil is promoted under a low temperature. When the compressor is reenergized, refrigerant in the evaporator flows into the compressor with a rush. This causes elements such as the leaf valve of the compressor to be damaged. The pumping down operation is carried out to prevent such an accident.
In pumping down operation, the electromagnetic valve arranged at the intake side of the evaporator is closed. Refrigerant existing between the evaporator and the compressor is collected on the discharge side of the compressor (redistribution of refrigerant).
The suction pressure lower limit valve Lv is determined in order to prevent compressor 13 from an abnormal operation. This lower limit value Lv is, in general, determined within the range of -0.5 kg/cm to -1.0 kg/cm against the prescribed pressure value (set value Sv).
In the above-described refrigerating apparatus, since the suction pressure lower limit value Lv is a fixed value the refrigerating apparatus has disadvantages, as described below.
In a small refrigerating load, e.g. in winter, the suction pressure of compressor 13 frequently falls below the lower limit value Lv. Therefore, the refrigerating apparatus frequently executes the refrigerant pumping down operation, rather than the refrigerating operation, in response to changes of the suction pressure of compressor 13.
In summer, since the actual temperature is relatively high, the refrigerating operation is carried out under a low suction pressure of the compressor below the set value Sv. On the other hand, since the discharge pressure of the compressor tends to increase because of a high actual temperature, the compressing ratio Pd/Ps (Pd: discharge pressure, Ps: suction pressure) of the compressor increases. Furthermore, the suction pressure of the compressor seldom falls below the lower limit value Lv during the refrigerating operation. As a result, the temperature of the compressor increases.
According to the conventional refrigerating apparatus described above, since the suction pressure lower limit value Lv of the compressor is set to a constant value regardless of the temperature of the atmosphere, energy consumption loss of the compressor is caused in winter, and the load of the compressor is increased in summer.